Since we had the 2016 Transit of Mercury yesterday, I thought it might be interesting for Gwendolyn to see some videos related to that event. These transits occur approximately 13-14 times per century, and basically involve the planet Mercury passing across the face of the Sun. The first transit of Mercury was observed in 1631 by Pierre Gassendi, some twenty or so years after Galileo Galilei pointed a telescope toward the skies. I took the opportunity to tell Gwendolyn a bit about Galileo, and the history of modern-day astronomy. Galileo, contrary to popular belief, did not invent the telescope, but he was the first to point this magnifying instrument toward the skies (we also looked briefly at the physics of lenses). What was so important about these first observations of transits was that the heliocentric model of the Solar System was still in its infancy, and seeing the perfect sphere of Mercury pass across the face of the Sun also consolidated the fact that Earth was, in fact, round, and not flat. In addition, some of the transit images we looked at were taken from orbiting spacecraft closer to the Sun, and so we could see the complex magnetic structure of the Sun's surface.